1. Field of the Invention
The present invention relates to an apparatus for performing planarization polishing of a substrate, and in particular to an apparatus for performing chemical-mechanical polishing of a substrate such as a semiconductor wafer, for planarizing the substrate.
2. Background Art
In earlier polishing apparatus, a substrate to be polished was held on the lower surface of a substrate carrier. Then, to polish the substrate, it was pressed against a polishing pad as a polishing slurry was supplied to the polishing surface of the pad, while establishing a relative motion between the substrate and the polishing pad. The substrate carrier was provided with a retainer ring to prevent the substrate from being dislodged from the lower surface of the substrate carrier during polishing. The substrate was surrounded by the retaining ring to hold it in place on the carrier
During polishing performed on the resilient polishing pad, however, excessive polishing occurred at the periphery of the workpiece, producing an effect referred to as ‘edge rounding.’ To suppress this edge rounding effect, the retainer ring was pressed against the polishing pad to flex it slightly downward at its periphery.
In conventional polishing apparatus such as described above, although pressing the retainer ring against the polishing pad solved the edge rounding problem, thus improving the surface finish precision of the polished workpiece, it also resulted in poor distribution of slurry over the surface of the workpiece being polished, which reduced polishing speed, thereby degrading productivity. This tendency was especially pronounced when the surface being polished was a metal film. Consequently, this conventional polishing apparatus was poorly suited for high-speed polishing, which requires that a large amount of slurry be supplied to the surface being polished.
Recently, polishing apparatuses that use a fixed abrasive instead of a polishing pad have been developed. Because a fixed abrasive experiences almost no shape deformation when a workpiece is pressed against it, it produces very little edge rounding, even in the absence of downward pressure from a retainer ring. When such a fixed abrasive is used for polishing in a conventional polishing apparatus, however, because the retainer ring is now being pressed against a fixed abrasive, the ring wears faster, and the shorter retainer ring service life causes a corresponding increase in cost.